Process for the manufacture of articles resistant to gaseous corrosion



tes-1.... 13,1942] assaeei THE ARTICLES RII'ISIRE'EAWJI.

- scams or to GASEOUS coa- Rene Peri-in, Paris, France, asslgnor to Societe dElectrochimie dElectro-Metallurgie et des Acieries Electrlques d'Ugine, Paris, France, a corporation of France No Drawing. Application April 22, 1939, Serial No. 269,483. In France June 2, 1934 13 Claims.

This invention relates to a process for imparting to articles of alloyed metal the property of improved resistance to corrosion by a given gaseous substance especially at high temperatures. It is particularly adapted for rendering iron and steel articles resistant to corrosion by gaseous oxygen and sulphur compounds at high temperatures.

The present application is a continuation-inpart of my application, Serial No. 23,524, filed May 25, 1935. s

It is known that certain alloying elements, such as aluminum, silicon, chromium and zirconium, when incorporated in suitable proportions in alloyed metals (especially in alloyed iron or steel) impart thereto a considerably increased resistance to chemical corrosion when exposed to gaseous substances such as oxygen or oxidizing mixtures (e. g. air, water vapor or carbon dioxide) or to gaseous sulphur compounds (e. g. sulphur dioxide or hydrogen sulphide) especially at high temperatures.

Protection against such gaseous corrosion is generally ascribed to the formation upon the surface of the alloyed metal, at the beginning of its utilization, of a film or coating of a metallic compound which is little or not at all attackable by the atmosphere in which the said metal is used; said film or coating serving to prevent said alloyed metal from being appreciably attacked subsequently by the corrosive constituents of said gaseous body. Metallic compounds having the properties required for this purpose are known to thos skilled in the art. It is known, for instance, that a coating of alumina, formed upon the surface of an article of aluminum, will protect the latter from attack by the oxygen of the air, because, when adherent and of sufficient thickness, it prevents the base metal from coming into contact with the air. Such protective coating may be constituted principally of chromium oxide or aluminum oxidewhen the article to be protected is made of an alloy of chromium or aluminum and is to be subjected in use to an atmosphere containing oxygen at high temperature. It is understood that the protection afforded bysuch a film or coating is due not alone to the chemical nature thereof, but also to its properties of adherence, compactness and thickness. The term base metal is used herein to desig- Heretofore a protective coating which fulfills the stated conditions has been obtained only by alloying with the base metal a very high proportion of alloying elements; which, besides involving excessive cost, has frequently the detrimental efiect of imparting to the alloyed metal properties at variance with those it should possess. This occurs in the case of ferrous alloys when it is necessary to add thereto a large proportion (30% to of chromium or 10% or' more of aluminum in order to render them corrosion-resistant to oxygen at high temperature, say of the order of 1000 C. Hence'it is very diflicult, and sometimes impossible, to regenerate such alloys (i. e. to subject them to a heat treatment for the purpose of restoring thereto properties which have deteriorated or disappeared in the course of mechanical operations, such as forging or the like) and consequently the range of application of such alloys is greatly restricted. Such chrome alloys, moreover, when maintained for a long time at high temperature, are liable to become fragile by reason of the growth of the grain of the ferrite.

It is obvious that, in the case of exposure to oxygen it is possible with an alloy of iron and a more readily oxidizable metal to form only the oxide of the more readily oxidizable metal by subjecting such alloy to a heating at high temperature at a partial pressure of oxygen of the order of that corresponding to the dissociation of the oxide of the more readily oxidizable metal at that temperature, since under these conditions the iron is practically not attacked. However,

" the use industrially of such extremely small partial pressures of oxygen involves dimculties so serious as to make it impracticable.

Moreover, the problem under consideration is not only to obtain at low cost a determined coating-composition, but also to produce a coating having th required properties of homogeneity, adhesiveness, compactness and thickness.

Contrary to previous practice and theory, the present invention makes possible the manufacture of corrosion resistant metallic articles of various shapes made of alloyed metals, especially ferrous alloys. This result is accomplished by adding to the base metal relatively small pronate the chief component of the alloy (iron or I steel for example) from the alloying metal (chromium or aluminum for example) which is used to protect such chief constituent from corrosion.

portions of alloying elements and then treating the alloyed metal articles by a heating treatment under operating conditions that are thoroughly practical and economical, as hereinafter more fully described. The quantities of alloying elements required for accomplishing the above stated result by the process of this invention are relatively much smaller than required for the production of corrosion resistant alloys by methods heretofore used. For example, by the treatment of this invention a steel containing only 12% chromium is rendered resistant to the oxidizing action of air when the article is heated to 1,000 C., whereas a chrome steel which has not been so treated must contain at least about 30% chromium, in order to make it equally resistant to oxidation under these same conditions. The amount of alloying element added to the ferrous base metal will vary according to the conditions under which the articles are to be used. Thus where chromium is the alloying metal and iron or steel the base metal and the article is intended to resist oxidation at 800' C. it was previously necessary to provide a chromium content of about 12%. By the treatment according to the invention a steel containing only 5.5% chromium is rendered resistant to the same extent to oxidation at 800 C.

This invention has for its essential object to produce upon an article made of an alloyed metal containing one or several alloying constituents a coating which will ailord effective protection against corrosion by exposure to a determined gaseous body at high temperature. In case the alloy to be treated contains'more than one alloying constituent, the said protective coating may comprise several compounds, corresponding respectively to the several alloying constituents. In any case but 'a single protective coating is, or need be, formed.

The present invention consists essentially in subjecting the article to be rendered corrosion resistant to the action, at a sufliciently high temperature, of an atmosphere containing the gaseous substance which the article is intended to resist in actual use or of an atmosphere containing a gaseous substance which when combined with the alloying metal gives rise to a coating capable of resisting to the gaseous atmosphere in which the article is to be actually used. If the article is to be used in an atmosphere containing oxygen, it is given a heating treatment in an atmosphere containing oxygen. The heating treatment is caried out at a temperature sufficiently high to cause a combination of the gaseous substance and the alloying element of the alloyed metal until a suificiently thick compact coating is obtained. The temperature of the heating treatment is preferably at or above the temperature to which the article is to be subjected in use. However, the temperature of the heating treatment need not be as high as the intended temperature of use but need be only sufficiently high to cause a combination of the gaseous heating treatment substance and the alloying element. The heating treatment is conducted in an atmosphere containing the gaseous substance at a partial pressure above the dissociation pressure of the compound of the gaseous substance and the alloying element but low enough to avoid the formation of such an amount of the corresponding compound of the base metal as would unduly impair the effectiveness of the coating. The alloying element of the alloyed metal which combines with said gaseous substance is such that the compound formed thereby has a dissociation pressure below that of the compound formed by the action of said gaseous substance upon the base metal; the treatment herent compact and homogeneous coating oisufflcient thickness is obtained.

Thus in order to obtain an article of chrome steel which is resistant to oxidation at high temperature (e. g. 1,000 0.), the process may be carried out advantageously by starting with a chrome steel having a chromium content insufflcient in practice to be resistant to corrosion by itself at that temperature (say a 12% content of chromium) and subjecting this steel to a temperature near 1,000 C. at a pressure of oxygen between a fraction of a millimeter and several millimeters of mercury, say between ,5 and 2 mm. of mercury for a suflicient time to produce a coating of chromium oxide thick enough to render the article corrosion resistant to oxygen at the temperatures and pressures under which it is intended to be used. The pressure of oxygen used during the heating treatment is kept insufficient to cause the formation of a coating rich in iron oxide, which coating would be permeable and would impair the effectiveness of the coating. If in the heating treatment just described the pressure of oxygen is mm. of mercury, it will ordinarily be sufficient to carry out the heating treatment for a period of about hours to produce a sumciently thick compact homogeneous and adherent protective coating which will withstand satisiactorily the oxidizing action of air when the heat treated article is intended for resisting oxidation by air at 1,000 C. The heating treatment described will produce a coating which is adherent and not detachable from the article. By examination by X rays it is found that this coating is constituted by CrzOs containing a small proportion of F8203. The thickness of this coating is less than mm. I

The length of time required for forming a coating of proper thickness varies, of course, with varying conditions and depends alsoupon the particular use for which the article under treatment is destined. Therefore, in each case all details must be determined experimentally. This. however, is readily done. The maximum of oxygen pressure is likewise determined experimentally by a series of trials, in which the pressure is varied in a regular and progressive manner, as well understood by those skilled in the art.

In the treatment of an article of steel alloyed with say 4% of aluminum, the process may be carried out in a manner substantially similar to that described above. Obviously, in all cases the maximum pressure of oxygen at the determined temperature, to which the alloyed metal should be subjected during the heating treatment according to this invention, will depend upon the conditions attending the use for which the particular article is destined. Where, for example, chromium is the alloying metal, the protective coating may contain more oxide of iron (wherefore a higher partial pressure of oxygen at a determined temperature is permissible in the treatment acording to this invention) when the article is intended for use at a lower temperature and with a lower pressure of oxygen. A protective coating may, for example, be formed at 1000 C. and with a partial oxygen pressure which is higher where the alloyed metal is to be used in air at 500 C. than when the same alloyed m tal is to be used at mom I It should be observed that the protective coating will in fact contain not only the compound of the alloying metal (e. g. chromium oxide) but alsoannanpropnrflinotthecompoundotthebaaemctal(e.l.imnmde):in cewhereofthecorroalonreslstaneeis. oicourse,decreaaed. Therefore, thelowerthetemperatureinwhkhthearticleis tobeuaedthehisherthem ofironoxide.

'lheconsiderahleoi'suchatreatmentashereinlsthatltpennilsthc utilization of articles made of alloys containing anflloyinstinmuchsmallerproportlonsthanheretotorerequiremwhichalloysue, therefore,muchlesscostlytlnnthosemade heretoioreforthesamenflrposaandwhichahove allaremorereadilyworkahlebothinthehot 'andinthemldconditlon. Thusasteelwithl2$ ofchromiumisveryeasytoworkandiseasily heattreatedtorestorepropertiesdiminishedhy worhngwhileahimnalloyofwkchromimn isferrltlcatallthemnxeoituuperahumih properties cannot be restored byheat treatment 'andhasmoreoverthetageoi tively high temperature according to this invention,thepartiaipressureofthegaseoussubstanceitispomihletocansethenatureofthe coating formed vely to vary also. It is thuspossihletousefinallypartialpof said gaseous substance whicharerelaiivehrhigh andoftheorderoipressurestowhichthetreated articleisto beusedunder industrial conditions. liencethefinalpartiaipressurestobeemployed inaccordance withtheinvcntionarehigherin thecaseofOandFe,thanthedissociafionpres- 7 sure or the oxide of the alloying element and that of FeO.

It is, therefore, possible to produce a coating oisuchanature (duetoitschemicalcornposh tion and to its adherence, iis homogeneity, its and its compactness) that the alloyed metal, when subjected to action of atmospheres common in industry, shows an extraordinarily increased corrosion-resistance. Moreover, the remarkable quality oi resistance to corrosion above described is attained with alloyed metals containing alloying elements (such as Al or Cr) in proportions very much lower than those required hithertoinordertoobtainequailyhigh.

corrosion-resistance.

For example, to obtain a suflicientiy thick coatingoioifideonanironorsteelallcycontaining Al, the alloyed metal is first subjected to a relatively low pressure of oxygen, which neverthelessisveryhighwithrespeettothedisscciaiion pressure of oxides such as A1203 (for example, 1.5-2 mm. of mercury); then, when the first coating is formed, the partial pressure of maybeincreasedslowlyandvelyt motmercury. Inthisway,andwitlnartalteringtheredstancetoconoslonotthecoaflngthe thickness thereof may be in a relativelyshorttlme.

Inasmuchastheparflalofoxygenis.

vcrylflxhrclafivetothemesur ht thetemperatureoftheh'eahnmtloftheoxide otthellhylngelementaddedtothebasemetal, theformaflonofacompactcoaflngofanomde otsufllcimtthicknwtobeconoslunlesistantto nowmmedlnlndustrymay beellfectedinatlvelyshorttime. Ashes been already explained, the proper conditions (temperature, pressure length of treatmentandthelikflrequiredforthenmdum tlon ofaproteciive coating havlngthedeslred mticaaretohedetermtnedhysuitahletats, regardbelnghadlneachcasetothepartlcular use for which the article under treatment is destined.

As a general rule, the following principles should be followed:

(1) 'lhepartialprmureofomenshouldhe lower, the lower the proportion of alloying elemmt added to the base metal.

(2) 'Ihetreatmentmayheelfected athlaher parflalpresurethelowerthetemperatureofflre treahnent itself; thus temperature and presm-e mayhecau'sedtovaryinversely.

(3) 'lheparfialpressureotoxygenshouldhe sumcientlyhiehandfhedurationofthetreatmentshooldbeslmicientlylongsothatacompactcoatingisobtaineiwhichisthickenough to aiford an us protecfinn agaimt subsequentcorrosion. Itisnotsuflieientinfachto ohtainacoatingofadeflnitenahneitbeingalso thatthiscoafingbecompact, adherent, andoismhclentthicknesstoinsurecorrosionresistance. The partial presure at which the teaianentis carried outand the duration of the will he, therefore, determined experimentalhineachmse.

When the coating is not sufliciently homogene-ous(l.e.inaplaneparalleltotheoutersurface 01 the coating),- as explained below, the

places at which a lackpf such homogeneity exists-- willbemanifestedhytheaofslitsor cracks. when the coating is not sufllciently adherent, "it has a tendency to separate from the base metal. when thecoating is not sufliciently compact, there is liability or the formation of cracksunder external influencw and hence of corrosion. When the coating lacks the requisite thickness, thereisariskofitsaccidentalremoval. I

It is to be understood in this connection that the homogeneity required for the p posm or this invention is not absolute or uniform homogeneity throughout the coating. What is required is that it should have a surface homogeneily; or, more precisely, the coating should be homogeneous in a plane which is parallel to the Outer surface of the article. If, therefore, we consider two points of the coating, both being atthesamedepthfromtheexternalsurface thereof, those points should show the same cornposiiicn and structure.

Two examples or treatments with an indication diameter of iron aluminum alloy containing 4% aluminum was heated to 1000 C. for a period of eight hours in an atmosphere of pure dry oxygen imder atmospheric pressure. It absorbed270 cc. oxygen calculated at C. and 760 mm. ofmer consisted in'maintaining the article for 72 hours at 1000 C. in an atmosphere of oxygen at a partial pressure of 2 mm. mercury. The article was then tested in the same manner as was the first article-namely, by heating it to 1000 0.

metal and will form a suiilciently thick compact homogeneous and adherent protective coating" on the article to protect it eilectively from the attack of the gaseous atmosphere to which the article is to be subjected in use. Another alloying element which I may use in addition to aluminum, chromium, silicon, and zirconium previously mentioned is beryllium.

The amounts oi alloying elements used will depend, among other factors, on the composition and temperature of the gaseous atmosphere in which the article is to be used. It also depends upon the amount of the particular alloying element which may be added to the parfor a period of 8 hours inan atmosphere-of pure ticular base metal without unduly adversely aidry oxygen under atmospheric pressure. Under these conditions of test. it absorbed only 5 cc. of oxygen, that is only it as much as the untreated artiele. Furthermore, the 5 cc. of oxygen absorbed also was less than that absorbed by 9 another article identical in every way but made of steel containing 10% of aluminum but not treated according to the present invention, when tested under identical conditions.

An article of an iron chromium alloy containmg 12% of chromium was tested for corrosion resistance with pure dry oxygen under atmospheric pressure at11000 C. for a period of 8 hours. It absorbed 420 cc. of oxygen. A like article of the same dimensions of the same composition was given a heating treatment according to the present invention which consisted in maintaining the article for 72 hours at 1000 C in an atmosphere of oxygen at a partial pressure of 2 mm. of mercury and was then tested by 85 heating it to 1000 C. for a period Of 8 hours in pure dry oxygen under atmospheric pressure. It absorbed only 10 cc. of oxygen in this testthat is to say only as much as the untreated alloy. The 10 cc. of oxygen which was absorbed was less than that absorbed by a like article identical in every way but made of an alloy containing 30% of chromium when tested under the same conditions but untreated according to the present invention.

Amongst its industrial applications for the production of articles intended to resist high temperatures without being rapidly destroyed by the gases of the atmosphere to which they are exposed, may be mentioned for example furnace sonable limits of a specification, the following 65 further explanation is given.

I have specifically referred to the use of chromium, aluminum, silicon and zirconium as alloying elements. The invention is not limited to these particular alloying elements but is applicable to other alloying elements'which form corrosion resistant compounds when subjected to gaseous atmospheres, which corrosion resistant compounds have dissociation pressures lower than that of the corresponding'compound of the base .15

fecting its physical properties and its ability to be worked into the desired shape. Considering chromium as the alloying element and iron or steel as the base metal for the production of articles intended to resist oxidation of air at atmospheric pressure and" at temperatures of about 1000 C., the amount of chromium is preferably about 12%. The amount of chromium may be reduced, if the temperature of use is lowered or the partial pressure of the oxygen in the air is reduced or the degree of corrosion resistance required for the article is less than that ordinarily encountered. The resistance to corrosion of a ferrous alloy containing chromium in any appreciable amount, say as low as 3%, is increased by treatment according to the present invention. As a general statement, I shall use for most purposesnot less than 12% of chromium in ferrous alloys intended for use in air under atmospheric pressure. The upper limit of chromium for ferrous alloys will probably not be above 30%, because to increase the chromium beyond this would be uneconomical or would adversely affect its fabricating properties too greatly. The range of chromium which will be used most generally in ferrous alloys is between about 12% and 17%.

With regard to'aluminum as'the alloying ele-- ment, the preferred percentage for ferrous alloys is about 4%. I prefer to use not less than 1.5% in most cases and not over 6%. The silicon may be as low as 2% or as high as 7% but is preferably between 3% and 4%. The other alloying elements, if used, may be employed in percentages substantially less than are required to produce the desired protection where the article is not subjected to a previous heating treatment in accordance with the present invention. Thus, as a general statement, it may be said that /z or V; of the usual amount of alloying element is required according to the present invention.

The temperature of the heating treatment for producing the protective coating is preferably at or near the temperature of intended use of the article. This temperature, however, may be materially less than the temperature of intended use of the article, so long as the heat treating temperature is high enough to form the corrosion resistant compound of the alloying element. This temperature, as previously pointed out, will dependon the pressure of the gaseous substance in the heat treating atmosphere which causes corrosion. As a general statement, it. may be. said that if the chromium ferrous article is to be used for resisting oxidation of air at atmospheric pressure and at 1000 C., the temperature of the oxygen containing heating atmosphere will not be less than 600 0. nor higher than The heating treatment temperature for aluminum containing ferrous alloy articles for use at temperatures of about 1000 C. in air at atmospheric pressure may be between 800 C. and 1200 C.

The time of the heating treatment for a ferrous alloy containing 4% of aluminum, in order to produce an article intended for use in air at atmospheric pressure and at a temperature of 1000 C, is generally between 10 and 100 hours where the partial pressure of oxygen in the heating atmosphere is 1 mm. of mercury. Generally the time of heating will be of the order of 80 hours. For a chromium containing ferrous alloy to be used under the same conditions, the time of heating treatment is generally also between 10 and 100 hours.

The partial pressure of oxygen in th heating atmosphere may be as low as 360 mm. of mercury or as high as 3 mm. 'of mercury, depending upon the heating treatment temperature, the proportion of alloying element and the intended use of the article.

It is to be emphasized again that the values which have been given are merely for illustrative purposes and it is not intended to limit the invention thereto.

The word article as used herein includes not only articles which have reached their final form but also articles which have not reached their final form.

The invention is not limited to the particular examples or to the preferred embodiments but may be otherwise embodied or practiced within the scope of the following claims.

I claim:

1. A process for improving the resistance of a metal to corrosion by a given gaseous substance, which comprises alloying with the said metal an alloying element which combines at high temperature with the gaseous substance to form a corrosion resistant compound having a dissociation pressure lower than that of the compounds of the base metal with said gaseous substance which are formed under the same heating conditions, and subjecting the alloyed metal to a heating pretreatment in an atmosphere contain ing the said gaseoussubstance at a partial pressure above the dissociation pressure of the said compound of the gaseous substance and the alloying element but below that at which the corresponding compound of the base metal forms in amount to materially impair the efiectiveness of the coating wanted for the contemplated use of the alloy and below the partial pressure of said gaseous substance to which the alloyed metal is to be subjected in actual use, and at a temperature at which the alloying element combines with the gaseous substance to form the above said corrosion resistance compound, the heating being prolonged until a compact homogeneous and adherent protective coating of substantial thickness is obtained on the alloyed metal.

2. A process for improving the resistance of a metal to corrosion by a given gaseous substance, which comprises alloying with the said metal an alloying element which combines at high temperature with the gaseous substance to form a corrosion resistant compound having dissociation pressure lower than that of the compounds of the base metal with said gaseous substance which is formed under the same heating conditions, said alloying element being added in an amount insufficient to give by itself satisfactory resistance to corrosion by the said gaseous substance under the intended conditions of use, and subjecting the alloyed metal to a heating pretreatment in an atmosphere containing said gaseous substance at a partial pressure above the dissociation pressure of the said compound of the gaseous substance and the alloying element but below that at which the corresponding compound of the base metal forms in amount to' materially impair the efiectiveness of the coating and below the partial pressure of said gaseous substance to which the alloyed metal is to be sub- .iected in actual use and at an elevated temperature at which the alloying element combines with the gaseous substance to form the above said corrosion resistant compound, the heating being prolonged until a compact homogeneous and adherent protective coating of substantial thickness is obtained on the alloyed metal.

3. A process for improving the resistance of a metal to corrosion by a given gaseous substance, which comprises alloying with the said metal an alloying element which combines at hightemperature with the gaseous substance to form a corrosion resistant compound having dissociation pressure lower than that of the compounds of the base metal with said gaseous substance which are formed under the same heat-' ing conditions, and subjecting the alloyed metal to a heating pretreatment in an atmosphere containing the said gaseous substance at a partial pressure above the dissociation pressure of the said compound of the gaseous substance and the alloying element but below that at which the corresponding compound of the base metal forms in amount to materially impair the effectiveness of the coating and below the partial pressure of said gaseous substance-to which the alloyed metal is to be subjected in actual use and at a temperature at which the alloying element combines with the gaseous substance to form the above said corrosion resistance compound, the heating being prolonged until a compact homo geneous and adherent protective coating of substantial thickness is obtained on the alloyedmetal, and progressively increasing the partial pressure as the heat treatment is continued.

4. A process for improving the rmistance of iron and steel to corrosion by oxidizing and sulphurous gases, which comprises alloying with a ferrous base metal an alloying element of the class consisting of chromium and aluminum, and subjecting the ferrous alloy to a heating pretreatment in an atmosphere containing the gas at a partial pressure above the dissociation pressure of any compound which forms by the reaction of the gas and the alloying element but below that at which the corresponding compound of the ferrous metal forms in amount to materially impair the efiectiveness of' the coating wanted for the contemplated use of the alloy and below the partial pressure of said gaseous substance to which the iron or steel is to be subjected in actual use and at a temperature at which the alloying element combines with the gas to form the above said corrosion resistant compound, the heating being prolonged until a compact, homogeneous and adherent protective coating of substantial thickness is obtained on the ferrous alloy.

5. A process for improving the resistance of iron and steel to corrosion by oxidizing and sulphurous gases, which comprises alloying with a ferrous base metal an alloying element of the class consisting of chromium and aluminum in an amount insuflicient to render the alloy resistant to corrosion by oxidizing and sulphurous gases under the intended conditions of use, and subjecting the ferrous alloy to a heating pretreatment in an atmosphere containing the gas at a partial pressure above the dissociation pressure of any-corrosion resistant compound which forms by a reaction of the gas and the alloying element but below that at which the corresponding compound of the ferrous metal forms in amount to materially impair theeilectiveness of the coating and below the partial pressure of said gaseous substance to which the iron or stel is to be subjected in actual use and at a temperature at which the alloying element combines with the gas to form the above said corrosion resistant compound, the heating being prolonged until a compact, homogeneous and adherent protective coating of substantial thickness is obtained on the ferrous alloy.

6. A process for improving the resistance of iron and steel to oxidation, which comprises alloying with the ferrous base, chromium in an amount insufllcient to render the alloy resistant to oxidation under the intended conditions of use, and subjecting the ferrous alloy to a heating pretreatment in an atmosphere containing oxygen at a partial pressure above the dissociation pressure of chromium oxide but below that at which iron oxide forms in amount to materially impair the effectiveness of the coating and below the partial pressure of oxygen in the atmosphere to which the iron or steel is to be subjected in actual use and at a temperature at which chromium combines with oxygen, the heating being prolonged until a compact homogeneous and adherent protective coating high in chromium oxide and low in iron oxide and of substantial thickness is obtained on th ferrous alby.

7. A process for improving the resistance of iron and steel to oxidation, which comprises alloying with the ferrous basenaluminum in an amount insuiiicient to render the ferrous base resistant to oxidation under the intended conditions of use, and subjecting the ferrous alloy to a heating pretreatment in an atmosphere containing oxygen, at a partial pressure abov the dissociation pressure of aluminum oxide but below that at which iron oxide forms in amount to materially impair the effectiveness of the coating and below the partial pressure of oxygen in the atmosphere to which the iron or steel is to be subjected in actual use and at a temperature at which aluminum combines with oxygen, the heating being prolonged until a compact homogeneous and adherent protective coating high in aluminum oxide and low in iron oxide and of substantial thickness is obtained on the ferrous 1 alloy.

8. A process for improving the resistance of iron and steel to oxidation, which comprises alloying with a ferrous base metal from 3% to 30% gen, the heating being prolonged until a compact homogeneous and adherent protective coating of chromium oxide of substantial thickness is obtained on the ferrous alloy.

9. A process for improving the resistance of iron and steel to oxidation, which comprises alloying with a ferrous base metal from 3% to 30% of chromium, and subjecting the so formed ferrous alloy to a heating pretreatment at a temperature between about 600 and 1100' C. in an atmosphere containing oxygen at a partial pressure above the dissociation pressure of chromium oxide but below that at which iron oxide forms in amount to materially impair the effectiveness of the coating and below the partial pressure of oxygen in the atmosphere to which the iron and steel is to be subjected in actual use, the heating being prolonged until a compact homogeneous and adherent protective coating of chromium oxide of substantial thickness is obtained on the ferrous alloy.

10. A process for improving the resistance of iron and steel to oxidation at high temperatures, which comprises alloying with a ferrous base metal from 1.5 to 6% of aluminum, and subjecting the so formed ferrous alloy to a heating pretreatment at a temperature of between 800 and 1200 C. in an atmosphere containing oxygen at a partial pressure abov the dissociation pressure of aluminum oxide but below that at which iron oxide forms in amount to materially impair the effectiveness of the coating and below the partial pressure of oxygen in the atmosphere to which the iron or steel is to be subjected in actual use, the heating being prolonged until a compact, homogeneous and adherent protective coating of aluminum oxid of substantial thickness is obtained on the ferrous alloy.

11. A process for improving the resistance of iron and steel to oxidation at high temperatures, which comprises alloying with a ferrous base metal about 3% to 30% of chromium, and subjecting the so formed ferrous alloy to a heating pretreatment at a temperature between about 600 and 1100" C. for 10 to hours in an atmosphere containing oxygen at a partial pressure above the dissociation pressure of chromium oxide but below that at which iron oxide forms in amount to materially impair the effectiveness of the coating, the partial pressure of the oxygen in the heating pretreatment atmosphere being between ,6 mm. and 3 mm. of mercury, the heating being prolonged until a compact homogeneous and adherent protective coating of chromium oxide of substantial thickness is obtained on the ferrous alloy.

12. A process for making steel containing about 12 to 17% chromium resistant to oxidation at temperatures of the order of about 1000" 0., which comprises subjecting the steel to a heating pretreatment at about 600 to 1100 C. in an atmosphere containing oxygen at a partial pressure of about .02 to 3 millimeters of mercury and above vthe dissociation pressure of chromium oxide but below that at which iron oxide forms in amount materially to impair the effectiveness of the protective coating which is formed and continuing such heat treatment until a compact homogeneous and adherent protective coating high in chromium oxide and low in iron oxide and of substantial thickness is formed on the surface of the steel.

13. A process for making steel containing about 1.5 to 6% aluminum resistant to oxidation at the protective coating which is formed and continuing such heat treatment until a compact homogeneous and adherent protective coating high in aluminum oxide and low in iron'oxide and of substantial thickness is iormed on the surface of the steel.

RENE PERRIN. 

